The present disclosure generally relates to optical fibers and, more specifically, to methods for making optical fiber preforms with low-index trenches.
Optical fibers with low-index trenches surrounding the core of the optical fiber may have improved bending performance and/or larger effective areas relative to comparable optical fibers which are formed without a low-index trench. Accordingly, the improved optical and physical properties of such fibers make them desirable for use in a variety of applications.
The formation of the low-index trench around the core of the optical fiber adds additional steps to the process of making an optical fiber preform and, as a result, adds significant costs to the process of making an optical fiber. Specifically, the low-index trench can be formed by depositing silica-based soot around the core portion of the optical fiber and doping the silica-based soot with a down-dopant which decreases the index of refraction of the consolidated silica-based soot (i.e., silica-based glass) relative to the core portion of the optical fiber. However, to prevent the contamination of adjacent portions of the preform with the down-dopant, the low-index trench is separately formed and consolidated directly on the core portion of the optical fiber after the core portion has been consolidated and prior to depositing the overclad portion of the fiber. Specifically, the core portion of the optical fiber preform is first formed and consolidated to solid glass. Thereafter, the low-index trench portion is deposited around the core portion and then doped and consolidated in a separate step to prevent the dopant from diffusing into the core portion and the overclad portion. Finally, the overclad is formed around the low-index trench layer and consolidated in yet another step.
In other processes for making optical fibers with less manufacturing steps, an overclad can be formed before the trench layer has been created and any consolidation steps associated with cladding region. Doping is then performed to create the low-index trench layer and the low-index trench and overclad layers are consolidated at the same time. While these approaches can save manufacturing time and cost, there are problems associated with doping the preform with the overclad in place. Notably, some of the doping precursor materials (e.g., SiF4) can be introduced into regions of the overclad outside of the intended low-index trench region. As a result, the optical properties of the fibers can be adversely affected by the presence of the index-lowering agents within the overclad region outside of the low-index trench region intended to be doped by the index-lowering agents.
Accordingly, a need exists for alternative methods of forming an optical fiber preform having a low-index trench region surrounding its core portion that are efficient and not prone to doping in regions of the preform outside of the low-index trench region.